Image display system

ABSTRACT

A black and white display device having pixels having a density three times higher in the horizontal direction as in the vertical direction.

This is a divisional of application Ser. No. 08/941,391 filed Sep. 30,1997 now U.S. Pat. No. 6,356,322, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image display system, and more particularlyto an liquid crystal image display system whose dependence on the visualangle and contrast are improved. This invention further relates to animage display system which conforms to the visual sense of viewers.

2. Description of the Related Art

Liquid crystal displays are widely used in notebook computers,electronic pocketbooks, car navigators and the like.

Among such liquid crystal displays, a twisted nematic mode liquidcrystal displays are most widely used. The twisted nematic mode liquidcrystal display generally comprises a pair of glass substrates havingtransparent electrodes, liquid crystal cells which are disposed betweenthe glass substrates and contain liquid crystal molecules twisted by 90°and a pair of polarizing plates which are disposed on the outer sides ofthe respective glass substrates with their directions perpendicular toeach other. Display of images is effected by controlling the electricvoltage applied across the substrates, thereby changing the orientationof the liquid crystal molecules so that the output of light passingthrough the liquid crystal cell changes. That is, when the electricvoltage applied is lower than a threshold voltage, the polarizationdirection of linearly polarized light passing through the light incidentside polarizing plate is rotated by 90° along twist of the liquidcrystal molecules and accordingly the light passing through the lightincident side polarizing plate can pass through the light emanating sidepolarizing plate which is disposed perpendicular to the light incidentside polarizing plate, whereby a bright spot is displayed. On the otherhand, as the electric voltage becomes higher than the threshold voltage,the major axes of the liquid crystal molecules begin to erect from themiddle portion between the electrodes and the optical activity begins todeteriorate. As the electric voltage is further increased, the opticalactivity is finally nullified, light passing through the light incidentside polarizing plate impinges upon the light emanating side polarizingplate without rotated and accordingly cannot pass the light emanatingside polarizing plate, whereby a dark spot is displayed.

Generally the liquid crystal displays are divided into a simple matrixand an active matrix according to the system for applying an electricvoltage to each picture element. Those most widely used at present are asuper twisted nematic mode liquid crystal display of the simple matrixand a thin film transistor liquid crystal display of the active matrix.However there have been known liquid crystal displays of other variousmodes.

In the conventional liquid crystal image display systems, there havebeen a problem that the gradation of the image changes depending on thevisual angle and the contrast is low. In order to overcome such aproblem, there have been proposed a liquid crystal image display systemin which a light guide layer and a light dispersing layer are providedon the cells in order to lessen dependence on the visual angle (JapaneseUnexamined Patent Publication No. 58(1983)-95378) and a liquid crystalimage display system in which parallel light is projected in order tolessen dependence on the visual angle and increase the contrast.

However, practically it is very difficult to completely collimate lightand actually light containing therein components directed in multipledirections projected onto the liquid crystal display, which results indeterioration in contrast since images by oblique components aresuperposed on the image by vertical components.

Further when light from a light source is completely collimated, theefficiency of utilization of light from the light source deterioratesand brightness is lowered.

Further it has been proposed to provide phase-contrast film on theliquid crystal to lessen change in gradation depending on the visualangle. However even this approach cannot completely overcome the problemof dependence on the visual angle.

Further conventional image display systems are not designed taking intoaccount characteristics of human visual response. That is, since theeyes are arranged in a horizontal direction, the visual response in thehorizontal direction is higher than that in the vertical direction andhumans can recognize in more detail in the horizontal direction. Howeverin the conventional image display systems, the density and the shape ofpicture elements are the same in the horizontal direction and thevertical direction. Accordingly, there sometimes happens thatinformation displayed in the horizontal direction is poor for the humanvisual response in the horizontal direction and information displayed inthe vertical direction is too much for the human visual response in thevertical direction.

SUMMARY OF THE INVENTION

In view of the foregoing observations and description, the primaryobject of the present invention is to provide a liquid crystal displaywhich is free from dependence on the visual angle and can display a highcontrast image.

Another object of the present invention is to provide an image displaysystem which conforms to the visual sense of viewers.

In accordance with a first aspect of the present invention, there isprovided a liquid crystal display system comprising a liquid crystalcell system formed by sandwiching liquid crystal between first andsecond transparent electrode substrates and a light source forprojecting substantially collimated light onto the first transparentelectrode substrate, an image being viewed from the second transparentelectrode substrate side, wherein the improvement comprises that

an optical compensation means is provided on each of the firsttransparent electrode substrate side and the second transparentelectrode substrate side of the liquid crystal cell system, and

a light dispersing layer is provided on the second transparent electrodesubstrate side of the liquid crystal cell system.

The “substantially collimated light” may include components which are ata slight angle to the parallel components.

In the liquid crystal image display system in accordance with the firstaspect of the present invention, the optical characteristics of thecomponents at a slight angle to the parallel components are corrected tosubstantially the same as those of the parallel components by virtue ofthe optical compensation means disposed on the light incident side andthe light emanating side of the liquid crystal cell system, whereby ahigh contrast image can be displayed. Further by virtue of the lightdispersing layer provided on the light emanating side of the liquidcrystal cell system, the visual angle-dependence is lessened.

The liquid crystal may be of various types such as twisted nematicliquid crystal.

In accordance with a second aspect of the present invention, there isprovided a liquid crystal display system comprising a liquid crystalcell system formed by sandwiching liquid crystal between first andsecond transparent electrode substrates and a light projecting means forprojecting light onto the first transparent electrode substrate, animage being viewed from the second transparent electrode substrate side,wherein the improvement comprises that

said light projecting means is disposed on the first transparentelectrode substrate side adjacent thereto and comprises a point lightsource disposed in a position where it is near the first transparentelectrode substrate and does not face the first transparent electrodesubstrate, a collimating optical system which collimates light emittedfrom the point light source to parallel light travelling in parallel tothe first transparent electrode substrate and a reflecting mirror whichis disposed facing the first transparent electrode substrate andreflects the parallel light to impinge upon the first transparentelectrode substrate in perpendicular thereto, and

a light dispersing layer is provided on the second transparent electrodesubstrate side of the liquid crystal cell system.

Said “position where the point light source is near the firsttransparent electrode substrate and does not face the first transparentelectrode substrate” is for example a position beside the reflectingmirror which is disposed facing the first transparent electrodesubstrate.

The collimating optical system may comprise, for instance, a Fresnellens disposed in perpendicular to the first transparent electrodesubstrate near thereto and a condenser lens which directs light emittedfrom the point light source toward the Fresnel lens.

An optical compensation means may be provided on the first transparentelectrode substrate side of the liquid crystal cell system with anotheroptical compensation means provided on the second transparent electrodesubstrate side of the liquid crystal cell system between the lightdispersing layer and the liquid crystal cell system.

The liquid crystal cell system may be divided into a plurality ofregions and a light projecting means may be provided for each region.That is, a plurality of light projecting means may be provided for oneliquid crystal panel. In this case, two or more point light sources mayshare one reflecting mirror.

The liquid crystal may be of various types such as twisted nematicliquid crystal.

In accordance with the second aspect of the present invention, bycollimating light emitted from the point source by the optical system,the light projected onto the first transparent electrode substrate canbe parallel light containing less oblique components. Further theefficiency of utilization of light from the light source can be high,whereby power consumption can be reduced.

Further since the light projecting means comprises a point light sourcewhich emits light in parallel to the substrates and a reflecting mirrorwhich reflects the light in perpendicular to the substrates, the lightprojecting means can be small in thickness.

By causing light containing less oblique components to impinge upon theliquid crystal cell system, a high contrast image can be displayed.Further by virtue of the light dispersing layer provided on the lightemanating side of the liquid crystal cell system, the visualangle-dependence is lessened.

In accordance with a third aspect of the present invention, there isprovided an image display system in which an image signal is reproducedas a visual image on a pixelized screen having a number of pictureelements arranged in horizontal and vertical directions, wherein theimprovement comprises that the density of the picture elements in thehorizontal direction is higher than that in the vertical direction.

It is preferred that the dimension in the vertical direction of eachpicture element be larger than that in the horizontal direction.

The image signal may be one on which picture element density conversionprocessing for causing the density of the picture elements in thehorizontal direction to be higher than that in the vertical directionhas been carried out, one read out in such a manner that the density ofthe picture elements in the horizontal direction becomes higher thanthat in the vertical direction, one read out on the basis of pictureelements whose dimensions are larger in the vertical direction than inthe horizontal direction, or one read out on the basis of pictureelements whose dimensions are larger in the vertical direction than inthe horizontal direction and at the same time whose density is higher inthe horizontal direction than in the vertical direction.

Preferably the density of the picture elements in the horizontaldirection is at least 1.2 times as high as that in the verticaldirection and more preferably three times as high as that in thevertical direction.

Preferably the dimension of each picture element in the verticaldirection is at least 1.2 times as large as that in the horizontaldirection and more preferably three times as large as that in thehorizontal direction.

The image display system may be of any type including those using liquidcrystal, CRT, FED or EL.

It is preferred that a maximum brightness exceeds 800 nit.

When the density of the picture elements in the horizontal direction ishigher than that in the vertical direction and the dimension in thevertical direction of each picture element is larger than that in thehorizontal direction, the response in the horizontal direction in a highfrequency range becomes higher and display conforming to the visualsense of viewers can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a liquid crystal imagedisplay system in accordance with a first embodiment of the presentinvention,

FIG. 2 is a fragmentary cross-sectional view showing one picture elementof the image display system shown in FIG. 1,

FIG. 3 is a schematic cross-sectional view of a liquid crystal imagedisplay system in accordance with a second embodiment of the presentinvention,

FIGS. 4A and 4B respectively show modifications of the image displaysystem shown in FIG. 3,

FIG. 5 is a view for illustrating an image display system in accordancewith a third embodiment of the present invention, and

FIG. 6 is a view for illustrating a modification of the image displaysystem shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a liquid crystal image display system in accordancewith a first embodiment of the present invention comprises a liquidcrystal cell system 10, first and second phase-contrast films (opticalcompensation means) 21 a and 21 b respectively disposed on oppositesides of the liquid crystal cell system 10, first and second polarizingplates 22 a and 22 b which are respectively disposed on the outer sidesof the phase-contrast films 21 a and 21 b, a light dispersing layer 23formed on the outer side of the second polarizing plate 22 b and a lightprojecting means 30 disposed on the outer side of the first polarizingplate 22 a.

In this particular embodiment, the liquid crystal cell system 10 is of atin film transistor type employing twisted nematic liquid crystal.

As shown in FIG. 2, which is a cross-sectional view showing one pictureelement of the liquid crystal cell system 10, the cell system 10 has alight incident side glass substrate 11 a on which amorphous silicon tinfilm transistors 12 and a transparent electrode 13 a are formed, and alight emanating side glass substrate 11 b on which black matrix 14,color filters 15 a, 15 b and 15 c and a transparent electrode 13 b areformed. The light incident side glass substrate 11 a and the lightemanating side glass substrate 11 b are located so that the transparentelectrodes 13 a and 13 b are opposed to each other and twisted nematicliquid crystal 16 is enclosed between the glass substrates 11 a and 11b.

The light projecting means 30 comprises a point light source 31 whichmay be, for instance, an incandescent lamp or a fluorescent lamp and amicro-louver 32. The micro-louver 32 permits only light components, outof light emitted from the point light source 31, which impinge upon themicro-louver 32 in perpendicular thereto to pass therethrough andabsorbs all the light components except the light components.Accordingly, the angular distribution of the light passing through themicro-louver 32 can be limited, whereby substantially collimated lightcan be projected onto the liquid crystal cell system 10.

The operation of the liquid crystal image display system will bedescribed, hereinbelow.

Light emitted from the point light source 31 and passing through themicro-louver 32 is substantially collimated. The substantiallycollimated light is polarized by the polarizing plate 22 a and the phasedifference in the light is compensated for by the phase-contrast film 21a. Then the light enters the liquid crystal cell system 10. Thetransmittance to light of the liquid crystal cell system 10 changesdepending upon the electric voltage applied across the transparentelectrodes 13 a and 13 b. That is, orientation of molecules of thetwisted nematic liquid crystal changes depending upon the electricvoltage, whereby transmittance to light is varied. The color filters 15a, 15 b and 15 c are red, green and blue filters, respectively. Bycontrolling the electric voltage applied to picture elements,intensities of light passing through the color filters 15 a, 15 b and 15c which are mixed to make color display are controlled. The lightpassing through the cell system 10 is compensated for by thephase-contrast film 21 b with phase difference generated while passingthrough the cell system 10, polarized by the polarizing plate 22 b andemanates from the light dispersing layer 23 dispersed in every directionby the light dispersing layer 23.

As can be understood from the description above, in the liquid crystalimage display system of this embodiment, the substantially collimatedlight emitted from the light projecting means is compensated for withphase difference and then is caused to enter the liquid crystal cellsystem 10. Then the light emanated through the light dispersing layer.Accordingly, an image which is high in contrast and low in dependence onthe visual angle can be obtained.

A liquid crystal image display system in accordance with an secondembodiment of the present invention will be described with reference toFIG. 3, hereinbelow. The image display system of the second embodimentis substantially the same as that of the first embodiment except thatthe light projecting means differs from that of the first embodiment.Accordingly the elements analogous to those of the first embodiment aregiven the same reference numerals and will not be described here.

As shown in FIG. 3, the light projecting means 30′ in the secondembodiment comprises a point light source 31 disposed in a positionwhere it does not face the liquid crystal cell system 10, an opticalsystem 32 which collimates light emitted from the point light source 31into parallel light and causes the parallel light to travel in parallelto the cell system 10, a reflecting mirror 33 which reflects theparallel light to impinge upon the cell system 10 in perpendicularthereto, and a reflector 34 which reflects light emitted from the lightsource 31 toward the optical system 32, thereby improving the efficiencyof utilization of light from the light source 31. The optical system 32comprises a Fresnel lens 32 b which collimates the light emitted fromthe point light source 31 and causes the collimated light to travelstraight in parallel to the cell system 10 and a condenser lens 32 awhich condenses the light toward the Fresnel lens 32 b.

The operation of the image display system of this embodiment issubstantially the same as that of the first embodiment.

As shown in FIGS. 4A and 4B, a plurality of light sources 31 and aplurality of optical systems 32 may be provided for one liquid crystalpanel 40. In this case, two or more point light sources 31 may share onereflecting mirror. For example, in the case shown in FIG. 4A, the liquidcrystal panel 40 is divided into four regions, that is, region a, regionb, region c and region d. The point light sources 31 for the regions maybe provided with separate reflecting mirrors. However the point lightsources 31 for the regions a and b may share one reflecting mirror andsimilarly the point light sources 31 for the regions c and d may shareone reflecting mirror. In the case shown in FIG. 4B, the liquid crystalpanel 40 is divided into two regions, region e and region f. The pointlight sources 31 for the regions may be provided with separatereflecting mirrors or may share one reflecting mirror.

In the first and second embodiments, various liquid crystals such asvertical array nematic liquid crystal using ECB effect, super twistednematic liquid crystal, ferroelectric liquid crystal, anti-ferroelectricliquid crystal, dispersion polymer type liquid crystal in which nopolarizing plate is necessary and the like may be used in place oftwisted nematic liquid crystal. The drive systems for these liquidcrystal are broadly divided into a single matrix system and an activematrix system. Though, in the embodiments described above, a tin filmtransistor system which is one of active matrix systems is used, otherdrive systems may be used.

An image display system in accordance with a third embodiment of thepresent invention will be described with reference to FIG. 5,hereinbelow.

FIG. 5 shows a display screen 110 and part of picture elements in thescreen 110 of the image display system in accordance with the thirdembodiment of the present invention. The screen 110 may comprise, forinstance, a liquid crystal panel. As shown in FIG. 5, in the screen 110,the density of the picture elements 105 in the horizontal direction (thedirection of arrow A) is three times as high as that in the verticaldirection (the direction of arrow B) and at the same time, the dimensionb in the vertical direction of each picture element 105 is three timesas large as the dimension a in the horizontal direction of each pictureelement 105.

The image signal which is to be reproduced on the screen 110 as avisible image may be one on which picture element density conversionprocessing has been carried out to conform the image signal to thedensity of the picture elements and the shape of the picture elements,or one read out on the basis of picture elements whose dimensions in thevertical direction are three times as large as those in the horizontaldirection and at the same time whose density in the horizontal directionis three times as high as that in the vertical direction.

For example, it is assumed that the number of picture elements 105 onthe screen 110 is 2400×600.

When an image signal made up of 1760×1760 image signal components is tobe reproduced on the screen 110, picture element density conversionprocessing for reducing the number of picture elements in the verticaldirection to one third while holding the number of picture elements inthe horizontal direction as it is is carried out on the image signal anda visible image is reproduced on the basis of the processed imagesignal.

When an image signal made up of 1024×1024 image signal components is tobe reproduced on the screen 110, picture element density conversionprocessing for doubling the number of picture elements in the horizontaldirection while reducing the number of picture elements in the verticaldirection to one half is carried out on the image signal and a visibleimage is reproduced on the basis of the processed image signal.

When the screen 110, where the density of the picture elements 105 inthe horizontal direction is three times as high as that in the verticaldirection and at the same time, the dimension in the vertical directionof each picture element 105 is three times as large as the dimension inthe horizontal direction of each picture element 105, is for white andblack display, the screen can be diverted to a conventional screen forcolor display by providing red, green and blue filters on each sets ofthree picture elements adjacent to each other in the horizontaldirection. In this case, each set of three picture elements functions asone picture element in the color screen and the color screen isisotropic in the horizontal and vertical directions.

In other words, when a conventional color display is diverted to a whiteand black display, an image display system which can display an imageconforming to the visual sense of viewers can be obtained.

When a screen for white and black display has a picture element densityin the horizontal direction higher than three times that in the verticaldirection and a ratio of the dimension in the horizontal direction tothat in the vertical direction of each picture element smaller than 1:3,a color display system which can display an image conforming to thevisual sense of viewers can be obtained by diverting the white and blackdisplay system to a color display system.

Though, in the embodiment described above, the screen has a pictureelement density in the horizontal direction three times as high as thatin the vertical direction and picture elements each of which has adimension in the vertical direction three times as large as that in thehorizontal direction taking into account diversion of the screen into ascreen for color display, the picture element density and the dimensionsof each picture element need not be limited to such.

When the picture element density in the horizontal direction is notlower than about 1.2 times that in the vertical direction, an imagedisplay conforming to the visual sense can be obtained. Further when thevertical dimension of each picture element is not smaller than about 1.2times the horizontal dimension, an image display conforming to thevisual sense can be obtained. Preferably the picture element density inthe horizontal direction is not lower than three times that in thevertical direction, and the vertical dimension of each picture elementis not smaller than three times the horizontal dimension.

For example, even if the horizontal dimension a′ and the verticaldimension b′ of each picture element 105′ are equal to each other asshown in FIG. 6, an image display high in response in the horizontaldirection can be obtained so long as the picture element density in thehorizontal direction is higher than that in the vertical direction. Theshape of the picture element need not be rectangular but may be circularor ellipsoidal. When the picture element is ellipsoidal, the major andminor axes may be considered to be the aforesaid vertical and horizontaldimensions, respectively.

When the maximum brightness is not lower than 800 nit, preferably notlower than 1500 nit, an image display excellent in visual contrast canbe obtained.

What is claimed is:
 1. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that: the density of the picture elements in the horizontal direction is at least three times higher than that in the vertical direction in which the dimension in the vertical direction of each picture element is larger than that in the horizontal direction.
 2. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is at least three times higher than that in the vertical direction in which said image signal is such on which picture element density conversion processing for causing the density of the picture elements in the horizontal direction to be higher than that in the vertical direction has been carried out.
 3. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is at least three times higher than that in the vertical direction in which said image signal is read out such that the density of the picture elements in the horizontal direction becomes higher than that in the vertical direction.
 4. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is at least three times higher than that in the vertical direction in which said image signal is read out such that picture elements whose dimensions are larger in the vertical direction than in the horizontal direction.
 5. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is at least three times higher than that in the vertical direction in which said image signal is read out such that picture elements whose dimensions are larger in the vertical direction than in the horizontal direction and at the same time whose density is higher in the horizontal direction than in the vertical direction.
 6. An image display system as defined in claim 2 in which the dimension of each picture element in the vertical direction is at least 1.2 times as large as that in the horizontal direction.
 7. An image display system as defined in claim 2 in which the dimension of each picture element in the vertical direction is at least three times as large as that in the horizontal direction.
 8. An image display system as defined in claim 2 in which said pixelized screen comprises a liquid crystal panel.
 9. An image display system as defined in claim 2 in which a maximum brightness of the picture elements is higher than 800 nit.
 10. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is three times higher than that in the vertical direction in which said image signal is read out such that the density of the picture elements in the horizontal direction becomes higher than that in the vertical direction.
 11. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is three times higher than that in the vertical direction in which said image signal is read out such that picture elements whose dimensions are larger in the vertical direction than in the horizontal direction.
 12. A black and white image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is three times higher than that in the vertical direction in which said image signal is read out such that picture elements whose dimensions are larger in the vertical direction than in the horizontal direction and at the same time whose density is higher in the horizontal direction than in the vertical direction.
 13. An image display system as defined in claim 1 in which said pixelized screen comprises a liquid crystal panel.
 14. An image display system as defined in claim 1 in which a maximum brightness of the picture elements is higher than 800 nit. 